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Re-entrant code problems with global data Code that can be entered by more than one thread of control is becoming increasingly common. Such code is used in programs for Microsoft Windows, the Apple Macintosh, and Linux and also in recursive routines. Re-entrant code creates the possibility that global data will be shared not only among routines, but among different copies of the same program. In such an environment, you have to make sure that global data keeps its meaning even when multiple copies of a program are running. This is a significant problem, and you can avoid it by using techniques suggested later in this section.

Microsoft needed to fix this problem, too, and threads were the answer A thread is a Windows concept whose job is to virtualize the CPU Windows gives each process its very own thread (which functions similar to a CPU), and if application code enters an infinite loop, the process associated with that code freezes up, but other processes (which have their own threads) are not frozen; they keep running!.

Code reuse hindered by global data In order to use code from one program in another program, you have to be able to pull it out of the first program and plug it into the second. Ideally, you d be able to lift out a single routine or class, plug it into another program, and continue merrily on your way.

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Threads are awesome because they enable Windows to be responsive even when applications are executing long-running tasks . Also, threads allow the user to use one application (like Task Manager) to forcibly kill an application that appears frozen because it is executing a long-running task . But as with every virtualization mechanism, threads have space (memory

consumption) and time (runtime execution performance) overhead associated with them . Let s explore this overhead in more detail now . Every thread has one of each of the following:

Global data complicates the picture. If the class you want to reuse reads or writes global data, you can t just plug it into the new program. You have to modify the new program or the old class so that they re compatible. If you take the high road, you ll modify the old class so that it doesn t use global data. If you do that, the next time you need to reuse the class you ll be able to plug it in with no extra fuss. If you take the low road, you ll modify the new program to create the global data that the old class needs to use. This is like a virus; not only does the global data affect the original program, but it also spreads to new programs that use any of the old program s classes.

Figure 4.16 General project settings. Pay close attention to the build-number formatter and the fail conditions. Builds are formatted with the revision number. In this case, you decide to fail if the build exceeds 10 minutes.

Thread kernel object The OS allocates and initializes one of these data structures for each thread created in the system The data structure contains a bunch of properties (discussed later in this chapter) that describe the thread This data structure also contains what is called the thread s context The context is a block of memory that contains a set of the CPU s registers When Windows is running on a machine with an x86 CPU, the thread s context uses about 700 bytes of memory For x64 and IA64 CPUs, the context is about 1,240 and 2,500 bytes of memory, respectively Thread environment block (TEB) The TEB is a block of memory allocated and initialized in user mode (address space that application code can quickly access) The TEB consumes 1 page of memory (4 KB on x86 and x64 CPUs, 8 KB on an IA64 CPU) .

Uncertain initialization-order issues with global data The order in which data is initialized among different translation units (files) is not defined in some languages, notably, C++. If the initialization of a global variable in one file uses a global variable that was initialized in a different file, all bets are off on the value of the second variable unless you take explicit steps to ensure the two variables are initialized in the right sequence.